Generally, solid-state imaging devices used in commercial digital still cameras, which have recently become increasingly popular in the market, must have a million or more pixels in order to achieve an image quality no less than that of a film camera, and those having three million or more pixels have recently been put to practical use. Additionally, the commercial digital still cameras are required to be reduced in size. Therefore, the number of pixels in the solid-state imaging devices must be increased without changing the chip size thereof, or the increase in the number of pixels and reduction in the chip size must be achieved at the same time.
Interline-transfer interlace-scan (IT-IS) CCDs are typically used as solid-state imaging devices with a large number of pixels. In this type of solid-state imaging device, when the number of pixels is increased without changing the chip size, the size of unit cells used for photoelectric conversion reduces accordingly. Therefore, the sensitivity and the amount of charge which can be carried, that is, a saturation signal level, are reduced. In order to compensate for this, various characteristic improvements have been made so that the number of pixels can be increased without causing the characteristic degradation due to the reduction in the size of the unit cells. However, if the number of pixels is further increased, the performance is inevitably degraded to some extent.
In order to fundamentally solve the above-described problem, solid-state imaging devices in which active devices, such as optical sensors and charge transfer electrodes, are arranged in multiple layers have been proposed, as described below.
(1) A method for increasing the sensitivity of a solid-state imaging device has been proposed in which a photoelectric conversion unit composed of polycrystal silicon or amorphous silicon is formed on a signal charge transfer unit so that the entire surface of the solid-state imaging device functions as a light-receiving surface and the amount of light received increases. However, since the mobility of electrons and holes in polycrystal silicon and amorphous silicon is lower than that in single-crystal silicon, a problem of afterimage or the like occurs. Accordingly, it is difficult to put this type of solid-state imaging device to practical use.
(2) A method has been proposed in which the thickness of a silicon substrate is reduced to about several tens of micrometers by back etching and an image is captured by causing light to enter optical sensors from the back. In this method, the amount of incident light increases since it is not impeded by transfer electrodes, and the sensitivity increases accordingly. However, since there is a limit to reducing the thickness of the silicon substrate, the application is limited to cases where infrared light, for which silicon has high transmittance, is received. In addition, it is difficult to increase the precision, and therefore this structure is not suitable for imaging devices with a large number of pixels which are required to be arranged at high density.
(3) A frame transfer (FT) CCD in which a single unit functions as both a photoelectric conversion unit and a charge transfer unit is also advantageous in that it has a large effective-charge-storage area. However, there is a problem in that the sensitivity reduces in a short wavelength region due to light absorption by transfer electrodes. In addition, the amount of dark current generated is large compared to an IT-CCD since a single unit functions as both the photoelectric conversion unit and the charge transfer unit, and there is a problem in that the S/N ratio is low.
In order to solve the above-described problems, a main object of the present invention is to provide a solid-state imaging device having a structure such that the number of pixels can be increased without increasing the size, and to provide a method for manufacturing the solid-state imaging device.